Chemical vapor sensing instruments are expected to become increasingly important in a variety of applications, including industrial hygiene, environmental clean-up, national defense and homeland security. Improved technology to reliably detect and identity potentially toxic or explosive volatile organic compounds is critically needed for these applications.
Many different approaches are being developed and refined throughout the academic, government and industrial/commercial research and development organizations. Current efforts have focused primarily on improving the portability of traditional analytical instruments such as gas chromatographs and optical spectrometers, and improving the sensitivity and reliability of array-based chemical sensors that measure gravametric, optical or electromechanical properties. The array-based chemical sensors are favored when low cost, portability, ease of operation and ease of maintenance are desired.
A sealed-deployable sensor system must be able to unambiguously detect an analyte of concern and provide an accurate measurement of the analyte of concern regardless of the presence of chemically similar interfering compounds. The most successful field-deployable sensor systems utilize cross-reactive chemical sensor arrays in which individual sensing elements are coated with materials that respond to broad classes of chemical vapors, with each sensor material being chosen to be sufficiently different from the other sensor materials so that the collective array of sensors will span a broad range of possible chemical properties.
A variety of organic, inorganic and organometallic materials have been tested as sensor materials. Notable examples include self-assembled monolayers, Langmuir-Blodgett films, clathrates, small organic molecules and polymers. Most functional commercial and/or prototype vapor sensing instruments use arrays of polymer-coated sensing elements. This is because polymer coatings can be selected to exhibit excellent sorptive properties for organic vapors, rapid diffusion if the polymer is above its glass-to-rubber transition temperature, reversible responses, and linear sorption isotherms over a large concentration range for low polarity vapors. The sensor coating materials may be selected to achieve a wide range of selectivities, such as by synthetic variation of the polymer structure.
In addition to the above properties, it is highly desirable that sensor-coating materials possess excellent processing characteristics. Specifically, mass production of reliable and inexpensive vapor sensing devices will require sensor-coating materials that are readily processable into thin, adherent films.
There are many polymers available that meet many of these requirements and offer other advantages. In particular, polymers can be made with a wide variety of functional groups (e.g., sensing groups) that may be incorporated directly into the polymer backbone during synthesis or appended to the polymer backbone after synthesis. Polymers may also be selected for a particular application based on appropriate molecular compositions and/or molecular architectures. Polymers may also be chosen based on the ability to cross-link, graft to substrates, or both. Many types of polymers are amorphous, i.e., either not crystalline or partially crystalline. Such polymers tend to be either rubbery or glassy and exhibit absorptive properties that may be advantageously employed for vapor sensing applications. Another advantage with using polymers for vapor sensing applications is that many polymers exhibit excellent stability and very low volatility at ambient conditions and/or elevated temperatures. Further, most polymers are easily processed into a variety of different forms. However, it is not easy to develop a simple polymer that will exhibit a combination of all these desired properties.
It is generally necessary and desirable to use an array of different sensors in a vapor-sensing device since it is impossible to achieve perfectly selective detection of any particular analyte using a single sensor. Any particular sensor will typically respond to several different analytes. However, any given sensor may respond to a particular set of analytes including some, but not all, of the analytes detectable by another sensor. Using a suitable array of sensors, it is possible to identify a single analyte based on a determination of the particular sensors in the array that respond to the analyte. By employing suitable calibration techniques, it may be possible to achieve quantitative analysis for a single analyte, and qualitative or quantitative analysis for a plurality of different analytes.
While vapor sensing devices employing an array of different sensors are generally required to achieve highly selective detection, it is desirable to use only a few different types of base polymers, and more preferably a single type of base polymer, for each of the sensors in the array, and modify the individual members of the array with different functional groups (sensing groups) to achieve a desired array of selective responses. This provides a simplified production process as compared with using a completely different polymer for each member of the array. However, polymers with every conceivable type of functional group are generally commercially unavailable. For example, polymers with hydrogen bond acidic functionalities, which would be useful for selective detection of hydrogen bond basic analytes such as chemical warfare nerve agents and explosives, and which otherwise meet the requirements for polymer detectors, are generally commercially unavailable.
There are currently three common categories of commercially available chemical vapor sensing devices utilizing polymer-coated sensing elements. A first category is devices comprising an array of acoustic wave sensors using quartz crystal microbalances (QCM), surface acoustic wave (SAW) devices, or flexural plate wave (STW) devices as the sensing transducers. For acoustic wave sensors, the generated signals are proportional to the mass of the vapor sorbed by each polymer coating on the surfaces of the device. A second category of commercially available array-based chemical sensors utilizes a chemiresistor transducer comprising an insulating polymer that is loaded with electrically conductive particles. In this device, vapor sorption swells the insulating polymer and increases the resistance through the polymer-conducting particle composite. Lastly, the third common category of commercially available array-based chemical sensors employs a sorbent polymer as the matrix for fluorescent dyes, such as Nile red. Vapor sorption alters the fluorescence signal from the incorporated dye molecules. In a minor variation of this, arrays have been prepared with various dyes in various polymers on the ends of fiber-optic bundles. It is also conceivable that a chemical vapor sensing apparatus could employ any combination of these three types of sensors.
The task of choosing and optimizing a chemically diverse array of polymer-coated sensing elements can be rationally accomplished through the use of solubility interactions and linear solvation energy relationships. It is believed that a sorbent polymer-based sensor array will collect the most chemical information if the polymer coatings in the array cover the full range of solubility interactions, including dispersion, dipole-dipole, and hydrogen-bond interactions. This approach requires a variety of different polymer coatings, including non-polar, polarizable, dipolar, hydrogen bond basic, and hydrogen bond acidic. Many polymer coatings exhibiting these types of solubility interactions are well known and commercially available, with the notable exception of the hydrogen bond acidic polymers. This presents a problem for at least two critically important applications for chemical vapor sensing apparatuses employing polymer-coated sensing elements. In particular, there is a need for the development of hydrogen bond acidic polymer-coated sensors for detection of nerve agents and explosives. In addition, there is a need for the development of hydrogen bond acidic polymer coatings to expand the chemical diversity of a sensor array, thus helping to optimize its ability to discriminate between various classes of vapors.
A large body of research on surface acoustic wave devices has demonstrated that incorporation of highly fluorinated alcoholic or phenolic functional groups into the sensing polymers is an effective way of maximizing hydrogen bond acidity. Concomitant minimization of basicity reduces the extent and strength of self-association, which would otherwise tend to lessen the driving force for interaction with basic vapors. Self-association is a major drawback to the use of carboxylic acid functional groups, which are good hydrogen-bond acids as monomers, but nearly always self-associate when in a condensed phase.
Strongly hydrogen bond acidic polymers also have intrinsically high polarity. Although this is a desirable feature in terms of chemical sensitivity, it poses a potential problem because highly polar function groups on polymers lead to higher glass-to-rubber transition temperatures than those of corresponding nonpolar polymers. Polymers having a glass-to-rubber transition temperature near or above room temperature (about 22° C.) typically act as diffusion barriers and provide unacceptably slow response times when used as coatings for chemical sensors.